The receptive field properties of visual cortical neurons are dynamic and can be altered both in space and time, in response to the pattern of activation to which the cell is exposed. Dynamical changes in receptive field properties are likely to contribute to several perceptual phenomena such as fill-in, afterimages, and contour integration. Until recently, the cellular mechanisms of these dynamic changes in cortical function have been difficult to examine. However, the application of intracellular recording techniques promises to determine at least some of the mechanisms underlying visual cortical plasticity. For example, through intracellular recordings we have shown that stimulation of cortical neurons with a high contrast stimulus results in the hyperpolarization of the membrane potential, apparently through the activation of intrinsic K+ conductances. This hyperpolarization is, at least in part, responsible for the shift in contrast response function that underlies contrast adaptation. Conversely, withdrawal of the high contrast stimulus results in a gradual depolarization of the cell, with opposite effects. In our preliminary experiments, we have observed that these changes in membrane potential not only change the amplitude of the visually evoked response, but also the shape of the receptive field. Membrane potential depolarization of cortical neurons results in an expansion of the receptive field size as well as an increase in the amplitude of visually evoked responses. Through this mechanism, presentation of an artificial scotoma (after stimulation of the receptive field) results in a gradual depolarization of the membrane potential and an expansion of the receptive field over a period of about 2-20 seconds. This time frame is remarkably similar to that associated with perceptual fill-in during the presentation of artificial scotomas in humans. Results from previous extracellular recording studies in vivo suggest that these perceptual phenomenon are mediated in large part through local and medium range (mm) horizontal interactions in the visual cortex. We suggest that regulation of membrane potential in cortical neurons may not only critically determine the response properties of the neuron under investigation, but also determine the interactions of cells in the local neocortical circuit, and contribute strongly to time and space-dependent dynamics of receptive field properties. Here we will test this hypothesis using intracellular recording studies in vivo in conjunction with visual stimulation. The ionic mechanisms of these effects will also be examined in detail with in vitro recording techniques. Through these studies, we will begin to achieve a cellular level understanding of visual perceptual mechanisms and fast (seconds) cortical plasticity.